The present invention relates to scroll-type fluid mover such as scroll-type vacuum pumps and compressors.
Scroll-type fluid mover, for example, scroll-type compressors have a movable scroll and a fixed scroll. Each scroll includes a base plate and a volute portion formed on the base plate. The volute portions cooperate to form a compression chamber. An eccentric shaft is formed on a drive shaft. The movable scroll is rotatably supported by the eccentric shaft. When the drive shaft rotates, the movable scroll orbits the axis of the drive shaft. Then, the compression chamber contracts from the periphery to the center of the volute portions, which compresses gas.
FIG. 6 shows a prior art structure for supporting a movable scroll with respect to the drive shaft. The structure of FIG. 6 is described as being prior art in Japanese Examined Publication No. 63-59032. In the apparatus of FIG. 6, an eccentric shaft 41 is formed on the drive shaft 42. The axis of the eccentric shaft 42 is displaced with respect to the axis of the drive shaft 42 in the radial direction by a distance equal to the revolution radius of a movable scroll 44. The drive shaft 42 is supported by a housing 48 of the compressor and a bearing 46. The movable scroll 44 includes a base plate 44a, a volute portion 44b projecting from the base plate 44a, a boss 43 formed on the opposite side of the base plate 44a from the volute portion 44b. The volute portion 44b cooperates with a volute portion 45b of a fixed scroll 45, which forms a compression chamber 47 between the scrolls 44, 45. The eccentric shaft 41 is inserted in the boss 43 and supports the movable scroll 44 through the boss 43. Accordingly, the eccentric shaft 41 supports the movable scroll 44 at a position outside of a working region R, which includes the volute portion 44b. In other words, the movable scroll 44 is supported at a position that is axially spaced from the compression chamber 47.
Centrifugal force is applied to the movable scroll 44 when it revolves. Also, compression reaction force generated by compressing gas in the compression chamber 47 is applied to the movable scroll 44. A resultant radial working force K, which combines the centrifugal force and the compression reaction force, is especially high in the working region R. However, the eccentric shaft 41 supports the movable scroll 44 at a position axially spaced from the region R. For this reason, the working force K applies an inclination moment to the movable scroll 44 with the supporting position of the eccentric shaft 41 at the center. For example, when there is a measurement error between the eccentric shaft 41 and the boss or between the volute portions 44b, 45b, the inclination moment inclines the movable scroll 44 with respect to the fixed scroll 45. Thus parts of the movable scroll 44 apply concentrated, localized forces to the fixed scroll 45. As a result, the smooth orbital movement of the movable scroll 44 is interrupted and the sealing of the compression chamber 47 between the scrolls 44, 45 deteriorates, thus causing rattling and gas leakage from the compression chamber 47.
To solve this problem, Japanese Examined Publication No. 63-59032 reveals the construction shown in FIG. 7. A movable scroll 44 has a boss 43 projecting on both sides of a base plate 44a. An eccentric shaft 41, which passes through the boss 43, is provided in the middle of a drive shaft 42. Accordingly, the eccentric shaft 41 supports the movable shaft 44 in a working region R, which includes the volute portion 44b. The drive shaft 42 has a first portion 42a and a second portion 42b, which are at opposite ends of the eccentric shaft 41. The first portion 42a is supported by bearings 46 and a compressor housing 48. The second portion 42b is supported by bearings 46 and a fixed scroll 45. Accordingly, the drive shaft 42 supports the movable scroll 44 at both sides of the working region R, or both sides of the compression chamber.
When a radial working force K based on centrifugal force and compression reaction force is applied to the movable scroll 44, the force K is received by the portions 42a, 42b of the drive shaft 42, which are located at both ends of the eccentric shaft 41. As a result, there is no inclination moment applied to the movable scroll 44, and the movable scroll 44 does not incline with respect to the fixed scroll 45.
To achieve smooth rotation of the drive shaft 42, the axes of the portions 42a, 42b of the drive shaft 42 must be precisely aligned and the axes of the bearings 46 must be precisely aligned. However, this increases the cost of production.
To insert the eccentric shaft 41, which is in the middle of the drive shaft 42, in the boss 43, at least one of the portions 42a, 42b of the drive shaft 42 must be separate from the eccentric shaft 41. After the eccentric shaft 41 is inserted in the boss 43, the separate part is fixed to the eccentric shaft 41. However, in this procedure, the number of parts and steps increase and the assembly work is difficult, thus increasing the manufacturing costs.